The present invention is related to a new support material to agglomerate mineral concentrates that permits the subsequent recovery of their metals of economical value via leaching. More specifically, it refers to the use of a support comprising of polymeric particles that are between 3 and 20 mm, non-porous, with a specific gravity less than 1, stable in highly corrosive environments such as strong acid solutions, resistant to abrasions, non-deformable at temperatures of up to 100° C., with the capacity for mineral concentrates to stably stick on the support's surface forming an agglomerate that can be easily available in a homogeneous pack, to allow for the spatial disposition of the concentrates in a fine layer on the support, via a physical process that allows said concentrate to agglomerate by using a rotary drum.
The reserves of oxidized minerals of high grade copper will continue to diminish in Chile and other parts of the world which in turn will cause the closure or diminish the operative capacity of the majority of the acid leaching operating plants within the next decade. As a result, other alternatives will be sought after such as solvent extraction (SX) and electrowinning (EW) installations required to concentrate and eletrodeposit the copper contained in the rich pregnant leach solutions (PLS) and charged electrolyte, respectively. In addition, due to the decrease in the exploitation of oxidized copper mineral resources, a significant increase in the exploitation of copper sulfide minerals can be foreseen in Chile. These minerals are normally treated by milling and flotation to generate high concentrations of copper sulfide minerals, which must be later treated by smelting. Currently, approximately 80% of the copper produced in Chile is via pyrometallurgy used in smelting plants. Concentrations of chalcopyrite (CuFeS2) are exclusively refined by this method. However, this method possesses inherent problems such as: high capital investment in which only large reserves are economically feasible for the exploitation of copper; elevated operating costs; and large volumes of SO2 gas emissions that require complex processes in order for the emission to be purified and finally SO2 is recovered in the form of sulfuric acid (H2SO4). If the sulfuric acid cannot be sold, it must be neutralized for it to be disposed of in an environmentally safe manner. Smelting plants, in turn, present problems with the emission of metallic dust, fugitive gases, and residual acidic solutions with high contents of Pb, As, Sb, Cu, Zn, Hg, Bi and Se that prevent the treatment of copper minerals with high concentrations of these contaminants, as a result, losing enormous reserves of copper sulfide minerals not available to be treated as it is in the case of enargite minerals (Cu3AsS4).
The largest proportion of mineralized copper as primary and secondary copper sulfides is found in the form of chalcopyrite. Table 1 shows in ascending order the sulfide minerals that are the most resistant to be leached. As it can be seen, chalcopyrite is in the second to last place and as a practical consequence it is the most recalcitrant copper mineral to be leached, requiring high temperatures and pressure to be dissolved.
TABLE 1Minerals according to its ascending resistance to oxidationPyrrhotiteFeSChalcociteCu2SCovelliteCuSTetrahedrite3Cu2S•Sb2S3BorniteCu5FeS4GalenaPbSArsenopyriteFeAsSSphaleriteZnSPyriteFeS2EnargiteCu3AsS4MarcasiteFeS2ChalcopyriteCuFeS2MolybdeniteMoS2
Due to the difficulties and the restrictions of the pyrometallurgy route, hydrometallurgy alternatives have been developed for the process of concentrates.
Among the diverse technologies of hydrometallurgy, bioleaching is an attractive alternative. Bioleaching is a solubilization methodology of metals starting from the oxidation of a complex mineral matrix, using the direct or indirect action of microorganisms. These microorganisms, in general, are capable of oxidizing some compounds such as iron ferrous and sulfur. The bacterial oxidation of minerals is a term applied to the microbiological solubilization of the contents of a mineral either to extract the valuable metal (i.e. bioleaching process) or to remove the contents of the mineral that accompany the metal of interest (i.e. biooxidation process) (1).
The majority of the microorganisms commercially used in the bioleaching process are acidophilic mesophilic bacteria which are mostly found in environments of high acidity and moderate temperatures (20-30° C.). However, in the last decades a series of studies and processes have been developed with extreme thermophilic microorganisms, isolated from hot springs and metallurgical processes, which live in environments of high quantities of salt and temperatures above 60° C. These microorganisms have shown to be more efficient in the dissolution of recalcitrant minerals than mesophilic bacteria.
There are various factors that affect the leaching and recovery of the metals of interests. The most important ones are: the type of reactor used, either heap, dump, stirred-tank or pressure leaching; the granulometry and dispersion of the fine material; the presence of clay and complex calcareous; the operating temperature of the reactor; the pH and the concentration of free acid in the leaching solutions; the type of mineral; and the ion concentration of potential inhibitors; among others.
From these references, a large quantity of studies and processes have been developed for the commercial application of bioleaching, allowing for the development of the designs of bioreactors such as the stirred-tank reactor or heap reactor.
The advantages of implementing systems of bioleaching and the recovery of metals of interests, among others, are that microbial extraction processes of metals present fewer risks for the environment than other metallurgy processes. The reason is that bioleaching processes do not require such intensive amounts of energy and produce sulfur dioxide nor other damaging gases, permitting that mineral concentrates with high grades of impurities be processed.
It is known that the leaching of chalcopyrite minerals, and in general primary copper sulfides and iron sulfides, is deflective to acid attack in heap leaching at an ambient temperature (moderate). This is due to the rapid passivation of the leached mineral by a layer of precipitates that forms on the surface of the mineral. These surface deposits significantly reduce the total kinetic recovery of copper in bioleaching. The formation of this passive layer greatly depends on the condition and variations of temperature, pH and oxygen concentration of the bed of the heap (2, 14). To avoid the formation of this passive layer, different leaching technologies for chalcopyrite, some physical-chemical nature, have been developed such as pressure leaching for copper concentrates. Through the application of high pressure of oxygen in an autoclave and a controlled adjusted temperature, a faster oxidation reaction can be achieved, in minutes to hours, and the formation of sulfur is present at the end of the process either in the form of elemental S° or sulfate SO4═. The disadvantage of these pressure reactors is their high capital and operating costs, and they have only been successfully used commercially in oxidation processes of minerals that contain gold, silver, and molybdenite. This is due to the fact that pure pressurized oxygen is used as the reagent in which enormous electrical energy is consumed, elevating the operating costs. Another technology, more chemical in nature, is one developed by Compañía Minera Michilla S.A., controlled by Antofagasta PLC, called CUPROCLOR (3). This technology is applied for the leaching of copper sulfides in heaps in the presence of excess chloride ions in the leaching solution, (90 g/L total chloride ions, 30 g coming from sea water used and the additional 60 g in the form of calcium chloride). The technology allows the presence of two redox couples Cu(II)/Cu(I) and Fe(III)/Fe(II) in solution and simultaneously avoids the formation of the passive layer of precipitates, resulting in high percentages of copper recovery (close to 95%). However, this model is not easily reproducible in transforming other oxidation leaching plants into sulfide processing plants.
Even though the bioleaching technology of primary copper sulfides at high temperatures offers an economical and environmentally friendly solution to the recovery problems generated by the formation of a passive layer on the surface of a particle during leaching, this technology has not been applied commercially for the bioleaching of neither chalcopyrite nor enargite concentrates, except in laboratory and stirred-tank reactor trail experiences, using thermophilic microorganisms such as in the BIOCOP process of Codelco Chile and BHP Billiton. However, the bioleaching process in a stirred-tank reactor has not been able to be developed into a commercially viable process because of the high investment costs required and the operational complexities demonstrated in the process trails (4).
In general, heap leaching of a layer of agglomerated concentrate on a support particle can be described as a process guided by a model of thin-layer leaching. As far as a good diffusion of oxygen is established in the bed and a high temperature in the interior of the heap leaching is maintained, to avoid the formation of jarosite precipitates on the surface of the particle to be leached, the leaching of copper sulfide minerals is made possible (5, 6). This system, in contrast to the stirred-tank, permits that the microorganisms capable of bioleaching stick to the surface of the mineral, forming a biofilm structured in base of a matrix of microorganisms and polysaccharides secreted by the microorganisms. Said matrix allows for a more effective bioleaching of the mineral particle (7).
In order to reach and maintain the required temperatures to achieve the bioleaching of copper sulfide minerals, different populations of iron-oxidizing and sulfur-oxidizing bacteria with different optimal growth temperatures are required to be present over time, attempting to optimize the growth rates of the different consortia (8). In relation to the use of microorganisms in leaching processes, there has been a change in perspective in the last decade due to the increase in complexity as an ever increasing diversity of microorganisms present in such environments. Today, the use of a microbiological consortia is being explored instead of a few primarily described “key” microorganisms, such as Acidithiobacillus ferrooxidans, A. thiooxidans or Leptospirillum (9,10), to which other microorganisms are added such as Acidianus brierley, A. thermosulfooxidans, Sulfubacillus thermosulfoxidans. This has happened, in part, to the significant contribution generated by the investigation of the microbial ecology field that has shown a wide diversity of microorganisms present in the natural environment of mining processes.
Currently, thermophilic microorganisms (bacteria or archaea), both moderate ones (with the capacity to grow at temperatures between 50 and 60° C.) and extreme thermophiles (with the capacity to grow at temperatures above 60° C.) having demonstrated to be capable of recovering copper from chalcocite minerals, have joined at a level of physical models and at a prototype and trail scale of copper sulfide bioleaching operations. The chemical reactions, carried out by metabolic activities of the microorganisms present in the biohdyrometallurgical environments of sulfide biooxidation, generate the physicochemical conditions necessary to elevate the temperatures inside the leaching reactors, observed in reactor types such as stirred-tank, heap or dump present in current mining operations.
Biolixiviación de calcopirita.
Chalcopyrite (CuFeS2) is the most recalcitrant copper sulfide to oxidation. Under the influence of A. ferrooxidans, the speed of oxidation of this sulfide increases significantly in comparison to purely chemical processes. Secondary copper sulfides, chalcocite (Cu2S), covellite (CuS), and bornite (Cu5FeS4) are more easily oxidized by direct or indirect action of bacteria. At an industrial level, the bioleaching technology has been applied in heaps (Chile, USA, Peru, etc.). Southern Peru has applied the technology for the recovery of copper in its low grade sulfides dumps of Toquepala. In Chile, Billiton and Codelco, in years past, carried out investigations to recover copper contained in arsenic minerals, a process named BIOCOP. Even though the process gave positive results from the kinetic point of view, it did not develop further into nor establish itself as a commercial process (11).
Bioleaching of Other Metal Sulfides.
Gold recovery: bacterial leaching is also used to break the sulfide matrix (mainly pyrite and/or arsenopyrite) that is found “trapped” in the gold-bearing particle, allowing the subsequent recovery of the gold through conventional cyanidation. In other words, the process is a pretreatment before the direct dissolution of the metal. Bactech, from Australia, has developed a process that uses moderately thermophilic bacteria for the treatment of sulfides and base metals known as the BACTECH process (12). The preliminary evaluations have reported the recovery of up to 98% of the gold contained in the mineral.
Zinc Recovery: the bacterial action in zinc sulfide has also been evaluated. Even though there are no known commercial plants, its application has enormous potential. Sphalerite is the most oxidizable zinc sulfide, influenced greatly by the presence of iron.
Lead Recovery: the bacterial leaching of galena originates the formation of PbSO4 that is insoluble in an acidic medium, a property that can be applied in the separation of some metal values contained in a lead ore.
Nickel Recovery: Nickel is leached from sulfides (pentlandite and millerita) and of iron ores in the presence of A. ferroocidans, which is 2 to 17 times faster than a purely chemical process.
Antimony Recovery: There is some work that reports the ability of At. ferrooxidans to oxidize stibnite (SB2S3) at pH 1.75 and at 35° C. In addition, At. thiooxidans is also reported to be capable to oxidize this sulfide.
Recovery of Rare Earth Mineral Metals: The rare earth metals are present in the crystalline portion of many sulfides and silicate minerals. In order to free the metals, it is necessary to oxidize the sulfides or destroy the matrix of the silicates. Literature reports the possibility of oxidizing a variety of these metals by using bacteria from the Acidithiobacillus genus, such as: gallium and cadmium present in sphalerite (the main transporter of these elements); of germanium and cobalt, of rhenium, selenium and tellurium, titanium and uranium, among others.
Previous Relevant Patents
Patent CN102230084 B describes a method of mineral treatment that includes stages of mineral grinding, mineral agglomeration with a adhesive, disposition in heaps and irrigation with sulfuric acid and a bacterial culture. It is important to highlight that the invention described in this patent does not include the agglomeration of the ground mineral on a support nor the maintenance of high temperatures during the leaching process.
U.S. Pat. No. 6,063,158 (MBX SYSTEMS, INC) describes an agglomeration method of mineral concentrate on a sphere shaped polyethylene matrix with a ring and a pin. The method also includes the use of T. thiooxidans to bioleach the agglomerated concentrate in systems of columns at ambient temperature.
U.S. Pat. No. 6,083,730 (Geobiotics Inc.) claims a method that consists of the agglomeration of a sulfide mineral concentrate on the surface of a thick mineral particle (volcanic rock, gravel or rock) with a size that ranges from 0.6 to 2.5 cm. This material is arranged in heaps for later bioleaching.
U.S. Pat. No. 8,491,701 claims a bioleaching method that consist of a first step of mineral agglomeration, the inoculation of the agglomeration with bacteria and their nutrients. The patent describes that at least part of the heap leaching consists of agglomerated material. However, it does not claim that there is an agglomeration method.
U.S. Pat. No. 8,119,085 describes a mineral leaching method that consists of grinding the mineral, agglomerating it in a agglomerator through the addition of water, a binder and an acid. Afterwards, heaps are formed which are irrigated with a solution that contains sulfur oxidizing bacteria.
U.S. Pat. No. 6,096,113 describes a bioleaching method in a closed tank that consists of treating part of the mineral with biooxidizing microorganisms. This pretreated mineral is then agglomerated on the non-treated mineral material. The agglomeration requires the use of drying materials and flocculants in a agglomeration device. This process is orientated to refractory minerals that contain precious metals, which are recovered from the oxidized product in the heap, through a heating process and then the addition of a leaching agent (such as cyanide).
U.S. Pat. No. 5,766,930 describes a method that mainly describes a method for bioremediate contaminated soils with organic substances in heaps, without shaking. The method consist of mixing the substrate, to be remedied, with layers of thick material selected among stones, pieces of brick, pieces of cement and plastic.
The GEOCOAT® technology consists of depositing a layer of sulfide concentrate on a support rock of a specified size, piling said material in heaps, irrigating it with solutions of acidic nutrients and providing air under low pressure to the base of the heap. After the biooxidation, the concentrate is removed from the rock support by wet sieving. The residue of the concentrate is then neutralized and subject to traditional methods of gold recovery. The support can be recycled.
Some of the disadvantages of this technology are that the substrate of the agglomeration is not completely inert to microbial action and of the acid. In addition, due to its mineral nature, the cost of the energy involved, both for the agglomeration and the recovery of the rock support, is by far superior to that of the technology of the present invention.
The present invention is different to the technologies developed by Geobiotics in that it considers an agglomeration substrate that is inert to the microbial activity and acid, is stable at high temperatures, is uniform in size, and has a low density. This last characteristic permits that the substrate be more easily recovered for its reuse.
From another perspective, the proposed technology resembles in part to the oxidation technologies of reduced compounds that in the moment they are oxidized, they become less dangerous. The reactions are similar to ones that occur in the oxidizing conversion of ammonium (NH4+) to nitrate (NO3−), for example in water treatment systems. In these systems, plastic particles are used as substrates on what microorganisms grow, forming biofilms. The solution to be treated is poured in the superior part of the reactor and percolates through the reactor while in a counter-current manner airflow enters from the base of the reactor.
This same principle has been suggested to be used in biomining in the proposed scheme of Vardanyan el al. (12), with the difference in this case that the authors propose to use substrates that are both organic and inorganic as systems of support, among which are mentioned calcium alginate, carrageenans, ceramic supports, activated carbon and porous matrices based on glass so that microorganisms with capacities to oxidize ferrous ions to ferric can bind. Afterwards, the bacteria bound to the support matrices are irrigated on their surface with a rich solution of ferrous ions while the reactor is aerated in a counter-current manner with air from its base. The irrigated solution percolates through the matrix/microorganism bed, accelerating the oxidization reaction and elevating the electrochemical potential of the solution, which can later be used for leaching sulfide and mixed copper minerals.